1. Field of the Invention
This invention relates to LED (Light Emitting Diode) based lamps and LED based light emitting signage. In particular, although not exclusively, the invention concerns a light emitting panel lamp and a back-light or light box for a light emitting sign.
2. Description of the Related Art
A lighting fixture commonly found in offices and commercial premises is a fluorescent lighting panel. Generally, such lighting panels comprise an enclosure housing one or more fluorescent tubes and a front diffusing panel. Typically, the diffusing panel is a translucent plastics material or a light transmissive plastics material with a regular surface patterning to promote a uniform light emission. Alternatively, a light reflective louvered front cover can be used to diffuse the emitted light. Such lighting panels are often intended for use in a suspended (drop) ceiling in which a grid of support members (T bars) are suspended from the ceiling by cables and ceiling tiles supported by the grid of support members. The ceiling tiles can be square or rectangular in shape and the lighting panel module is configured to fit within such openings with the diffusing panel replacing the ceiling tile.
White light emitting LEDs (“white LEDs”) are known in the art and are a relatively recent innovation. It was not until high brightness LEDs emitting in the blue/ultraviolet (U.V.) part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip generates blue light and the phosphor material(s) absorbs a proportion of the blue light and re-emits light of a different color typically yellow or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor material provides light which appears to the eye as being nearly white in color.
Due to their long operating life expectancy (of order 30-50,000 hours) and high luminous efficacy (70 lumens per watt and higher) high brightness white LEDs are increasingly being used to replace conventional fluorescent, compact fluorescent and incandescent bulbs. Today, most lighting fixture designs utilizing white LEDs comprise systems in which a white LED (more typically an array of white LEDs) replaces the conventional light source component. Moreover, due to their compact size, compared with conventional light sources, white LEDs offer the potential to construct novel and compact lighting fixtures.
Co-pending US patent application publication No. US 2007/0240346 (filed Aug. 3, 2006) disclose a back-lit lighting panel which utilizes blue/U.V. emitting LEDs instead of white LEDs. One or more phosphor materials are provided on, or incorporated in, a light transmissive window overlaying the back-light housing the LEDs. An advantage of providing the phosphor remote to the LED is that light generation, photo-luminescence, occurs over the entire surface area of the panel. This can lead to a more uniform color and/or correlated color temperature (CCT) of emitted light. A further advantage of locating the phosphor remote to the LED die (i.e. physically separated from the LED die) is that less heat is transferred to the phosphor, reducing thermal degradation of the phosphor. Additionally the color and/or CCT of light generated by the panel can be changed by changing the phosphor panel (window).
Edge-lit lighting panel lamps are also known in which light is coupled into the edges of a planar light guiding panel (waveguiding medium). The light is guided by total internal reflection throughout the volume of the medium and then emitted from a light emitting face. To reduce light emission from the rear face of the panel (i.e. the face opposite the light emitting face), the rear face will often include a light reflective layer. Moreover to encourage a uniform emission of light one or both faces of the light guiding panel can include a surface patterning such as a hexagonal or square array of circular areas. Each circular area comprises a surface roughening and causes a disruption to the light guiding properties of the light guiding panel at the site of the area resulting in a preferential emission of light at the area.
An advantage of an edge-lit lighting panel lamp compared with a back-lit panel lamp is its compact nature, especially overall depth (thickness) of the lamp which can be comparable with the thickness of the light guiding panel making it possible to construct a lamp of order 15-20 mm in depth. However, a disadvantage of edge-lit lighting panels is that they have a lower luminous efficacy compared with a back-lit arrangement due to light losses within the light guiding medium, losses in coupling light into the medium and losses in extracting light from the medium. Additionally as with back-lit lighting panels the light emission is not truly uniform over the light emitting face. For example there can be “hot spots” along the edges that correspond to the position of the LEDs and a darker region at the centre of the panel.
Co-pending U.S. patent application Ser. No. 12/183,835 (filed Jul. 30, 2008) discloses an LED based edge-lit light emitting panel in which a pattern of optical features (discontinuities) is provided on at least one face of the light guiding medium which are configured to reduce a variation in emitted light intensity over the light emitting surface of the panel. The pattern of features can be configured in dependence on the light intensity distribution within the light guiding medium. To reduce light losses associated with coupling into the light guiding medium the corners of the light guiding medium are truncated and light coupled into the truncated corners. Although such a pattern of features can reduce the variation in emitted light intensity since the panel is edge-lit the luminous efficacy can still be lower than a back-lit arrangement.
Co-pending U.S. patent application Ser. No. 11/827,890 (filed Jul. 13, 2007) describes an edge-lit lighting panel which utilizes blue emitting LEDs instead of white LEDs in which a layer of one or more blue light excitable phosphor materials is provided on the light emitting face of the light guiding panel. A proportion of the blue light emitted from the light emitting face of the panel is absorbed by the phosphor material(s) and one or more other colors of light emitted by the phosphor. For general lighting applications the lamp is configured such that the blue light from the LEDs combined with the phosphor generated light produces an illumination product that appears white in color. Since light generation (photo-luminescence) occurs over the entire light emitting surface area of the panel this can lead to a more uniform color and/or CCT of light emission. However, such a lighting panel still has the intrinsic losses associated with coupling light into the light guiding medium and extracting light from the panel resulting in a lower luminous efficacy compared with a back-lit arrangement.
In addition to general lighting applications back-lit lighting configurations are extensively used for light emitting signage, such as smaller format bill boards, in which a light transmissive display surface overlies the opening of the light-box enclosure. Often the display surface is in the form of an image printed on paper in which the paper acts a light diffuser and the printed image acts as a light transmissive color filter. Where the sign comprises symbols, characters or simple devices as opposed to complex images it is known to use colored acrylic, polycarbonate or other plastics materials to form the required image.
Co-pending patent application Ser. No. 11/714,711 (Publication US 2007/0240346) filed Jun. 3, 2007 discloses a light emitting sign which utilizes a blue light back-light and in which one or more phosphor materials are provided on the display surface and configured to generate a desired character, symbol or device of a selected color. An advantage of such a sign compared with one in which the display surface acts as a color filter is that the intensity and/or color saturation of emitted light is much greater.